In World System Teletext, a quantity of teletext information to be displayed as an entity on a television screen is termed a page. The pages are grouped together to form "magazines", and up to eight magazines may be provided, each of which may include up to 100 pages. All of the pages which are available for display are transmitted in a recurrent cycle, with or without up-dating page information, as appropriate. A teletext television receiver includes a teletext decoder and any page selected by a viewer for B display is acquired by the teletext decoder from the cyclic transmission the next time it occurs therein and is stored in the page memory of the teletext decoder until replaced by a subsequently acquired page.
Because of the large number of pages which are transmitted each page is only transmitted typically once every 20 to 30 seconds so that when a particular page is selected a waiting time of up to 30 seconds can be expected. This waiting time is unacceptable to many users. The waiting time can be reduced by adding extra memory in the teletext decoder in which additional pages can be captured, and if a user happens to select a pre captured page, the waiting time is eliminated. Many broadcasters now send additional information, e.g. Full Level One Features (FLOF) in the U.K. or Table of Pages (TOP) in Germany, to instruct the decoder as to which pages are to be pre-captured. These pages are usually those the broadcaster guesses the user will want to see next and are typically regulated to the current display page. A system operating in accordance with this principle is currently available and is referred to as "Fastext", and defines 4 pages to be pre captured. If a user makes page requests at random, it is unlikely that the decoder will have pre-captured the page due to the small amount of memory which is currently used.
Teletext decoders like ECCT (Philips integrated circuit SAA 5243) and IVT (Philips integrated circuit SAA 5246) are limited to storing up to 8 pages in memory and the controlling software does not have many options when determining which pages to capture and where to map them. Higher performance decoders such as DVTB (Philips integrated circuit SAA 9042) can capture up to 64 pages in memory but the store management task becomes very complex requiring a large amount of software and this places a high burden on the so-called I.sup.2 C bus as the controlling microprocessor used with DVTV needs to communicate very frequently with the decoder integrated circuit.
A so called Background Memory concept has been proposed which seeks to alleviate the waiting time experienced in teletext decoders. Such a background memory concept is disclosed in EP-A2-0333029. In a teletext decoder provided with a background memory it is arranged that the last N teletext data packets which are transmitted, a teletext data packet being, for example, the teletext data which corresponds to a row of a teletext display page, are stored in the background memory, (N depending on the size of the background memory). As each new packet arrives it overwrites the oldest packet currently stored, thus continuously updating the memory. When a page request is made, the contents U of the background memory are scanned rapidly to determine whether the required page is stored therein, and if it is, it is immediately transferred to the page memory of the teletext decoder for display purposes. Although it may have taken up to 30 seconds for those packets to be transmitted and stored in the background memory, scanning of the memory can be achieved in under a second with the result that if the requested page is in the background memory, the waiting time is virtually eliminated.
Thus, the normal teletext decoder page memory no longer needs to be partitioned into separate page stores, a single page memory only being required, and the microprocessor is relieved of the task of managing a multi-page store, thereby relieving the burden on the I.sup.2 C bus. In principle, the background memory technique works with any size of memory, the more memory that is provided the more data packets can be stored, thereby increasing the possibility of a requested page being present in the background memory, and increasing the probability of the waiting time being eliminated. The waiting time is completely eliminated if sufficient background memory is available to hold the data packets of all the transmitted pages.
However, the background memory concept suffers from a number of disadvantages. Firstly, background memory needs to be of relatively large size in order to store as many data packets as possible. Secondly, when the background memory is scanned, the scan starts from the data packet which has been stored for the longest period of time and continues to the last received data packet. With a large background memory, this can result in the required page being present in the background memory more than once and the first occurrence of the page which is detected by the scan will not necessarily be the most up-to-date version of the particular page, the page possibly having been updated before it was subsequently re-transmitted and stored for the second time in the background memory. Also, the requested page may be a rotating page where a sub-page X is followed by a sub-page Y, both of which would be stored in the background memory. When the background memory is scanned, sub-page X would be found and transferred to the page memory for display. Once scanning is completed or stopped, the teletext decoder reverts to looking at the "live" teletext data and the next sub-page which would be received and displayed would be sub-page Z, resulting in the user missing sub page Y. A related problem arises when the technique of "advanced" page headers is used in which an isolated page header is transmitted in advance of the actual page. This is sometimes done to improve transmission efficiency. When the background memory is scanned, the scan automatically stops after detecting the first occurrence of the page header, but this will not be followed by the actual page corresponding to that page header. Consequently the page would not be seen.